The radio frequency (RF) pulsed field of MRI can couple to an implanted lead in such a way that electromagnetic forces (EMFs) are induced in the lead. The amount of energy that is induced is related to a number of complex factors, but in general, is dependent upon the local electric field that is tangent to the lead and the integral of the subsequent electric field strength along the lead. In certain situations, these EMFs can cause currents to flow into distal electrodes and through the electrode interface to body tissue. It has been documented that when this current becomes excessive, overheating of the lead and the adjacent body tissue can occur. There have been cases of damage to both myocardial tissue and neurological tissue. In some cases, damage severe enough to result in loss of therapy, ablation, brain damage, limb amputations, and the like have occurred.
Magnetic resonance imaging (MRI) is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also used for interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes, and the like. An absolute contra-indication for pacemaker or neurostimulator patients means that these patients are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas.
Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a patient who may have any one of a number of active implantable medical (AIMD) devices. The literature indicates a number of precautions that physicians should take in such cases, including limiting the power of the MRI RF pulsed field (Specific Absorption Rate or SAR level), programming the pacemaker to fixed or asynchronous pacing mode, and then careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers or other AMDs after an MRI procedure, sometimes occurring many days later. Moreover, there are a number of papers that indicate that the SAR level is not entirely predictive of the heating that would be found in implanted leads or devices. For example, for magnetic resonance imaging devices operating at the same magnetic field strength and also at the same SAR level, considerable variations have been found relative to heating of implanted leads. It is speculated that SAR level alone is not a good predictor of whether or not an implanted device or its associated lead system will overheat.
There are three types of electromagnetic fields used in an MRI scanner. The first type is the main static magnetic field designated B0, which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the commonly available MRI units in clinical use. Some of the newer research MRI system fields can go as high as 11.7 Tesla or more.
The second type of field produced by magnetic resonance imaging is the pulsed RF field, which is generated by the body coil or head coil of the MRI instrument. The body coil is generally used to change the energy state of the protons and elicit MRI signals from tissue. The RF field is typically homogeneous in the central region and has two main components: (1) the electric field is circularly polarized in the actual plane; and (2) the B1 field, sometimes generally referred to as the net magnetic field in matter, is related to the electric field by Maxwell's equations and is relatively uniform. In general, the RF field is switched on and off during measurements and usually has a frequency of between about 21 MHz to about 500 MHz depending upon the static magnetic field of the MRI scanner. The frequency of the RF pulse for hydrogen scans varies by the Lamour equation with the field strength of the main static field where: RF PULSED FREQUENCY in MHz=42.56 (STATIC FIELD STRENGTH IN TESLA). MRI scanners employing NMR-like functionality are also capable of detecting ions with different gyromagnetic constants.
The third type of electromagnetic field is the time-varying magnetic gradient fields designated GX, GY, GZ, which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 2-5 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the operation of the MRI scanner.
The power deposited by RF pulses during MRI is complex and is dependent upon many factors including, power (Specific Absorption Rate (SAR) Level), duration time of the RF pulse, transmission frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. The amount of resultant heating also depends upon the volume of tissue imaged, the electrical resistivity of tissue and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the AMD and the length and trajectory of its associated lead(s). For example, even whether a pacemaker is positioned in the left or right pectoral will affect the amount of EMF that is induced into a pacemaker lead system. In addition, the routing of the lead and the lead length are also very critical as to the amount of induced current and heating that would occur.
Variations in the device lead length and implant trajectory can significantly affect how much heat is generated. A paper entitled, “HEATING AROUND INTRAVASCULAR GUIDEWIRES BY RESONATING RF WAVES” by Konings, et al., Journal of Magnetic Resonance Imaging, Issue 12:79-85 (2000), does an excellent job of explaining how the RF fields from MRI scanners can couple into implanted cardiac leads. The paper includes both a theoretical approach and actual temperature measurements. In a worst-case, they measured temperature rises of up to 74° C. after 30 seconds of scanning exposure. The contents of this paper are incorporated herein by reference.
Distal electrodes can be unipolar, bipolar, multipolar and the like. It is very important that excessive RF current not flow at the interface between the lead distal tip electrode or electrodes and body tissue. In a typical cardiac pacemaker, for example, the distal tip electrode can be passive or of a screw-in helix type as will be more fully described. In any event, it is very important that excessive RF current not flow at this junction between the distal tip electrode and, for example, into surrounding cardiac or nerve tissue. Excessive current at the distal electrode to the tissue interface can cause excessive heating to the point where tissue ablation or even perforation can occur. This can be life-threatening for cardiac patients. For neurostimulator patients, such as deep brain stimulator patients, thermal injury can cause permanent disability, debilitating comas or even be life threatening. Similar issues exist for spinal cord stimulator patients, cochlear implant patients, and the like.
A very important and possibly life-saving solution is to be able to control overheating of implanted leads during an MRI procedure. A novel and very effective approach to this is to first install parallel resonant inductor and capacitor bandstop filters (BSF) at or near the distal electrode of implanted leads. For cardiac pacemakers, these are typically known as the tip and ring electrodes. One such bandstop filter (BSF) solution is disclosed in U.S. Pat. No. 7,363,090 to Halperin et al. as well as U.S. Pat. Nos. 7,945,322, 7,853,324 to Stevenson et al., U.S. Pat. No. 7,899,551 to Westlund et al., U.S. Pat. No. 7,853,325 to Dabney et al. and patent application publication number, 2008/0049376 to Stevenson et al., all of which are assigned to the assignee of the present invention and are incorporated by reference herein.
Other types of component networks may also be used in implantable leads to raise their impedance at MRI frequencies and, therefore, reduce RF induced heating. For example, a series inductors may be used as a single element low pass filter. When positioned within an active MRI environment, the inductance of these component networks tends to increase at high frequencies, such as the RF pulsed frequencies of a typical MRI scanner, which inhibits RF lead heating. Component networks such as these are disclosed in U.S. Pat. No. 5,217,010 to Tsitlik et al. the contents of which are incorporated herein by reference.
U.S. Pat. No. 7,363,090 shows resonant L-C bandstop filters placed at the distal tip and/or at various locations along the medical device leads or circuits. These L-C bandstop filters inhibit or prevent current from circulating at selected frequencies of the medical therapeutic device. For example, for an MRI system operating at 1.5 Tesla, the pulsed RF frequency is 63.84 MHz, as described by the Lamour Equation for hydrogen. The L-C bandstop filter is generally designed to resonate at or near 63.84 MHz, thus creating a higher impedance that establishes an ideal open circuit in the lead system at that selected frequency. For example, the L-C bandstop filter when placed at the distal tip electrode of a pacemaker lead will significantly reduce RF currents from flowing through the distal tip electrode and into body tissue.
However, a drawback associated with these filtering solutions is that they generally take up space within the lead, particularly at the distal end of the lead. In many cases, it is difficult to incorporate these filtering solutions within a small diameter lead or a lead comprising a multitude of electrodes such as a neurostimulator. For example, a neurostimulator lead may comprise 6, 12, 16 or more electrodes, each of which may require a filter to reduce RF heating. In addition, implantable medical devices with intricately small leads, such as deep brain stimulators, make incorporating a component filtering solution prohibitively difficult.
Accordingly, there is a need for reducing RF heating in leads of implantable medical devices such as neurostimulators comprising a multitude of electrodes. Furthermore, there is a need for reducing RF heating in leads of implantable medical devices in which the leads are prohibitively small for use of component based RF filtering solutions. The present invention fulfills these needs and provides other related advantages.